The present invention relates to a method for calibrating an ophthalmic lens drilling machine, the machine including a drilling tool; an ophthalmic lens support associated with a first coordinate system; and a programmable tool guidance means associated with a second coordinate system expressing command coordinates which define a target drilling point. In the method the following successive steps are carried out. A template is placed on the support, the template having pre-applied markings defining a third coordinate system related to the template, such that the third coordinate system is made to substantially coincide with the first coordinate system. The template is drilled in at least one predetermined point corresponding to a target point defined by predetermined command coordinates, such that a real drilling point is obtained.
FIG. 1 shows schematically an ophthalmic lens drilling machine of a known type, which essentially comprises a support 2 on which a lens can be mounted and fixed for grinding, a drilling tool 3 which can be moved in a controlled way with respect to the support 2, and means 11 for guiding the tool 3.
The support 2 is shown schematically in the form of a receptacle enabling ophthalmic lenses of different shapes to be fixed with respect to the frame, in a fixed coordinate system O1, X1, Y1 associated with the support 2. The support 2 is provided to hold the ophthalmic lens in a support plane which is assumed to be horizontal. The reference axes X1, Y1 are therefore assumed to be horizontal.
The support 2 which is shown is a receptacle having an internal shape complementary to that of an adapter, of the type conventionally used to fix the lens on the movable arm of a grinder. An adapter of this kind is fixed, by gluing for example, to one of the faces of the lens. The receptacle 2, which is intended to receive an adapter of this type by insertion, has an indexing shape 2A complementary to an indexing shape of the adapter, which enables the lens to be orientated on the support 2, and thus with respect to the frame of the machine 1. The indexing means 2A thus define the orientation of the support 2 and of the frame of the machine, in other words the coordinate system O1, X1, Y1.
The drilling tool 3 is defined as being a tool which removes material around an axis, assumed in this case to be vertical (orthogonal to the axes X1, Y1), in the thickness direction of the lens, over a virtually point-like region of the lens or one having an area much smaller than the area of the lens. The term “drilling” can denote a conventional operation of drilling with a drill bit, resulting in the formation of a hole with a substantially circular cross section, or else an operation of “notching”, resulting in the formation of a notch in the edge of the lens, or any other type of more complex milling.
The guidance unit 11 for guiding the tool 3 are provided to move the tool 3 according to a machining task to be carried out on a lens placed in the machine. For this purpose, these guidance unit 11 comprise driver 13 adapted to move the tool 3, and controller 15 for controlling the driver 13, adapted to deliver to the driver 13 a command signal C corresponding to the machining task to be performed. The controller 15 is programmable means; it is provided to store a certain number of control laws with parameters set according to the shape and position of the drilling to be carried out. Thus the sequence of movements and operations executed by the tool 3, defined by the command signal C, is a function of the shape and position parameters supplied to the input of the controller 15. These parameters are indicated in FIG. 1 by the reference F (shape parameters) and by the references X, Y (position parameters). The position parameters X, Y are expressed in the second frame reference associated with the guidance unit 11, this virtual coordinate system theoretically coinciding with the first coordinate system O1, X1, Y1 related to the support 2.
FIG. 2 shows an ophthalmic lens 21 of generally rectangular shape, having a center marking O3 and axis markings X3, Y3 on one of its faces.
The center O3 represents the optical center of the lens 21, and the axis X3 represents its optical axis. The purpose of the marking of the axis Y3, perpendicular to the axis X3 in the general plane of the lens 21, is essentially to define the optical center O3 at its intersection with the axis X3.
When an adapter is centered on an ophthalmic lens blank for grinding, the center of the adapter coincides with the optical center O3 of the blank.
Thus, after the grinding operation which results in the production of the lens 21 in its finished form, when the lens 21 with its grinding adapter is placed on the support 2 for drilling in the machine 1, the center of the support O1 theoretically coincides with the optical center O3 located by the axis markings X3, Y3 on the leans 21.
If a hole is then to be drilled in the lens 21 with the drilling machine 1, the position parameters X, Y and the shape parameter F must be supplied to the controller 15, as mentioned above. For example, in order to create a virtually point-like circular drilled hole, the position parameters X, Y consist of the coordinates of the center M of the drilled hole. The coordinates X, Y, which are expressed in the second coordinate system associated with the guidance means 11, theoretically represent the coordinates of the center of drilling M in the coordinate system related to the lens, in other words the third coordinate system O3, X3, Y3.
When the drilling is actually carried out, it will be found that the real center of drilling (or real drilling point) Mr is offset with respect to the theoretical center of drilling (or target drilling point) M, as defined by the coordinates X, Y in the third coordinate system O3, X3, Y3.
This situation is shown in FIG. 3, in which the profile of the lens 21 and its markings defining the coordinate system O3, X3, Y3 are shown in solid lines, and the indexing shape 2A and the associated coordinate system O1, X1, Y1, as positioned with respect to the lens 21 when the latter is placed in the drilling machine 1 on the support 2, are shown in broken lines. The real centre of drilling Mr is also shown on the lens 21 in solid lines, and the theoretical centre of drilling M is shown in broken lines.
For reasons explained below, this offset is expressed by the coordinates dX, dY in one of the three pre-defined coordinate systems, which is assumed to be any one of these coordinate systems.
As a general rule, the offset of the real drilling points with respect to the theoretical drilling points is explained by the fact that the three coordinate systems defined above do not coincide exactly. On the one hand, the second coordinate system, associated with the guidance unit 11 and taken as the reference, for example, of the neutral position of the tool 3, is not exactly locked to the first coordinate system O1, X1, Y1 related to the support 2. This is due to the manufacturing tolerances and to the wear of the mechanical components used in the adjustment of the neutral position of the tool, to the tolerances and wear of the mechanical components of the driver 13, and to the intrinsic inaccuracies of the control elements used in the feedback control of the position of the tool 3, for example. On the other hand, the third coordinate system O3, X3, Y3 related to the lens 21 does not coincide exactly with the first coordinate system O1, X1, Y1 related to the support 2. This is due, in particular, to the inaccuracy, even if very small, of the positioning of the adapter on the lens, and the inaccuracy of the fixing of the adapter to the support 2, resulting, for example, from the manufacturing tolerances of these parts and from the possible deformation of the adapter during the preliminary grinding operation.
It should be noted that the offsets generally found in drilling machines between the theoretical and the real drilling points tend to indicate that there is no significant angular offset between the different coordinate systems. Consequently, in the description of the present invention, it is assumed that these coordinate systems are offset only with respect to translation, and that their horizontal axes, on the one hand, and their vertical axes, on the other hand, are parallel. This has been illustrated in FIG. 3, between the first coordinate system O1, X1, Y1 and the third coordinate system O3, X3, Y3.
For drilling machines used at present, it is therefore necessary, before the first use of the machine, to estimate the offset between the real drilling points and the theoretical drilling points, and to calibrate the machine so as to introduce a correction of the control laws into the controller 15. These calibration operations can be renewed periodically thereafter throughout the service life of the machine.
The correction which is introduced takes the form of a change of variables. For example, the position parameters taken into account for the calculation of the command C are X+dX, Y+dY, in place of the input parameters X, Y.
In the prior art, these calibration methods are implemented on the basis of a “manual” measurement of the offset produced by the uncalibrated machine. In the prior art, an operator uses the uncalibrated machine to drill a succession of virtually point-like circular holes in a template, such as an ophthalmic lens, and measures the positions of these drilled holes on the template by means of a caliper gauge. The operator then deduces the offset of each drilled hole with respect to the theoretical drilling points, and introduces a corresponding correction into the programmable machine guidance means. This correction can, for example, take into account a mean of the offsets found over all the measurement points.
This method has two principal drawbacks, namely the low accuracy of the measurement of the offset (of the order of a 10th of a millimeter), and the considerable time taken for the operation.